Phase modulation



April 13, 1943. M. e. CROSBY 2,316,155

PHASE MODULATION I Filed July 26, 1940 MOOULAT/A/G POTENUALS INVENTORMURRAY OSBY Qmy ATTORNEY CARR/ER SOU 06' I0 i 3 CARR/ER W VE SOURCEPatented Apr. 13, 1943 2,316,155 PHASE MODULATION Murray G. Crosby,Rlverhead, N. Y., assignor to Radio Corporation of America, acorporation of Delaware Application July 26, 1940, Serial No. 347,587

4 Claims.

This application concerns a new and improved phase modulation means ofthe type wherein an element of a phase shifter is varied in value toproduce a variable phase shift and consequently produce phasemodulation. A phase shifter utilizing inductance and resistance isemployed so that the resistance may take the form of a tube plateresistance which may be modulated in accordance with the signal voltage.Phase modulation has been accomplished heretofore by controlling thevalue of a circuit element at signal frequency to thereby control thephase of current therein. In my system Iuse a mid-tapped inductance anda resistance and vary the resistance. My circuit will produce a greaterphase shift for a given amount of resistance variation than certainknown systems of this nature. My system also makes a greater range oftotal phase variation possible. One of its most outstanding advantagesis the fact that the phase shift is accomplished without amplitudechange. This means that the phase modulation will not be accompanied byconcomitant amplitude modulation which must be limited oil by means of alimiter following the modulator stage.

In describingnry invention, reference will be made to the attacheddrawing wherein;

I Figs. 1 and 2 each show a modification of a phase modulator arrangedin accordance with my invention; while Fig. 3 illustrates the essentialelements of the phase shifter and is used with the vector diagrams ofFig. 4 to illustrate the operation of my system.

In the phase modulator of this disclosure a phase shifter such as shownin Fig. 3 is utilized. This phase shifter shifts the phase of voltage Elby an amount depending upon the magnitude of the reactances of Ll and L2and resistance R. The vector diagram of this type of phase shifter isshown in Fig. 4. The voltage to be phase-shifted is applied as El. Elcauses a current to flow through Li and R so that a reactance drop XLlIwill appear across points O--A of Fig. 3 and a resistance drop RI willappear across terminals O--C of Fig. 3. A reactance drop XmI will appearacross terminals -3 of Fig. 3 by virtue of the mutual inductive couplingwhich exists between LI and L2. Voltage El is the resultant of thereactance and resistance drops XLII and RI. The output voltage of thephase shifter E2 is the resultant of the mutual reactance drop XmI andthe resistive drop RI. It can be seen that El is different in phase byan amount 0 depending upon the-- relative values of the resistivevoltage drop RI and the reactive voltage drops XLlI and XmI.Consequently, by varying the value of R, the phase may be varied betweenzero and almost 180 degrees.

From the-vector diagram of Fig. 4, it can be seen that as R is variedbetween zero and a value equal to the reactive impedance, the phaseshift, 0, will be varied between zero and degrees and the amplitude ofthe output voltage, E2, will not vary since E2 is always equal to El sothat there is no variation of the attenuation with modulation.Consequently, phase modulation is possible without the introduction ofconcomitant amplitude modulation.

The phase shifter of Fig. 3 is adapted to use as a phase modulator byutilizing a resistance at R which can be controlled at a modulationpotenial rate. If the phase of the current is to be keyed or modulated,the value of the resistance R may be varied by keying signals whichshunt a portion of R or, all of R by a second resistance to change itsvalue in accordance with the signal. In a preferred embodiment theresistance R is comprised of the resistance of a vacuum. tube asillustrated in Figs. 1 and 2. Hence, by applying modulating potentialsto the grid of the vacuum tube, the phase of the voltage transmittedthrough the phase shifter is varied in accordance with the modulatingpotentials since, as is well known, the plate resistance of a vacuumtube may be varied byvarying the grid potential. Circuits having thesefeatures are shown in Figs. 1 and 2..

In the circuit of Fig. 1 constant carrier energy is supplied from sourceIll by transformer II to the grid ll of amplifier 20. The output circuitconnected with the anode 22 of the ampliilertube Zll comprises a tunedcircuit 4 wherein the amplified wave appears, and from which it is fedto the phase shifter consisting of inductance 26 and the resistancebetween the plate 30 and cathode 34 of tube 36. The phase-controlledoutput of the phase shifter is fed to the control grid ill of theamplifier tube 42 and thence to tuned circuit 44. Energy from tunedcircuit 44 is fed to power amplifier l8 and antenna 9. Power amplifier48 may contain frequency multipliers and limiters as is known to the artof phase modulation.

Modulating potentials are applied to Jack 3i and then throughtransformer 32 to the grid 35 oftube 36. Bias is furnished to the grid35 of tube 36 by means of battery 31.

It is well known that the variation of the grid voltage on a tube variesits plate resistance. Consequently, the application of modulating potentials to the grid 35 of tube 38 will vary the plate resistance of tubeIt and, in turn, vary the phase In the circuit r Fig. 2 the resistancebetween the anode 50 and cathode 52 of a diode tube 54 is used for thevariable resistance of the phase shifter. This diode resistance isvaried by applying the modulating potentials through Jack 3|,transformer 32 and the radio-frequency choke RFC. Condenser II is ablocking condenser to keep the plate voltage from amplifier tube 20 frombeing applied to diode 54. Battery 5 1 biases the diode to amid-position on its resistance-voltage characteristic so that the dioderesistance may be modulated at a higher or lower value than thismid-position value, It can be'easily seen; how the elements of thecircuit of Fig. 2 can be substituted for the corresponding elements ofFig. 1, and further description of this modification is believedunneces- SB 'Y.

When the frequency of the carrier to be phase modulated is high, so thatthe plate capacity of tube36 becomes a low-impedance shunt in comparisonto the plate resistance, tuned circuit TC may be added to tune out the"capacity. This tuned circuit is tuned to the carrier frequency so as tobe resistive to that frequency. TC is addedby closing switches S and SI.

Asdescribed in my United States application Serial No. 36,473 filedAugust 16, 1935, now United States Patent #2,104,318 issued January 4,1938, several of these phase modulators may be cascaded in order toproduce an increased or more faithful degree of phase modulation.

What is claimed is:

1. 'Means'for shifting the phase of alternating current including aninductance, a variable resistance having one terminal connected to a.

point on said inductance and a second terminal connected to a point oflow alternating current voltage, said variable resistance havinginherent reactance, connections for impressing voltages the phase ofwhich is to be shifted on said injductance and resistance,connections'for deriving voltages, the phase of which may be varied byvarying the value of said resistance, from said 1 inductance andresistance, and connectionsincluding the inherent reactance ocf saidresistance in a circuitparallel tuned to the frequency of said voltages.

2; In a phase modulation system a source of wave energy the phase ofwhich is to be modulated in accordance with control potentials, an

inductance, a variable impedance having one terminal coupled to a pointon said inductance and a second terminal coupled to ground or equivalentpotential, said impedance having inherent reactance, a circuit couplingsaid source to one terminal oi said inductance and to said secondterminal of said impedance, connections for deriving phase shiftedcurrent from said inductance and said impedance, a control circuit forvarying the value of said variable impedance in accordance with saidcontrol potentials, and connections including the inherent reactance ofsaid impedance in a circuit parallel tuned to theifrequency of said waveenergy.

3. In a phase modulation system, a source of wave energy the phase ofwhich is to be modulated in accordance with modulating potentials, aninductance, an electron discharge device having an anode, a cathode anda controlgrid, a connection between the anode of said device and a pointon said inductance, means for impressing wave energy from said sourcebetween a second point on said inductance and the cathode of saiddevice, means for deriving phase modulated wave energy from a thirdpoint on said'inductance and the cathode of said device, a source ofmodulating potentials, means for impressing modulating potentials fromsaid source on the control electrode and cathode of said device, and acircuit tuned to the frequency of said wave energy connected between theanode and cathode of said device.

4. In a. wave length modulation system, a source of alternating currentthe wave length of which is to be modulated, a source of modulatingpotentials, an inductance, anelectron discharge device having an anodeelectrode and a cathode electrode, said device having inherent reactancewhich appears between said electrodes, a lead coupling one of saidelectrodes to a point on said inductance, couplings for impressingcurrent from said source to be modulated on said inductance and theother electrode of said device, a circuit for modulating the impedanceof said device in accordance with modulating potentials from saidsource, connections for deriving modulated current from said inductanceand said other electrode of said device, and connections including thereactance inherent in said device between its anode and cathode in acircuit parallel tuned to the frequency of the current of said source ofcurrent.

' MURRAY G. CROSBY,

